The structure of an active matrix OLED or AM-OLED is well known. It comprises:                an active matrix containing, for each cell, an association of several thin film transistors (TFT) with a capacitor connected to an OLED material; the capacitor acts as a memory component that stores a value during a part of the video frame, this value being representative of a video information to be displayed by the cell during the next video frame or the next part of the video frame; the TFTs act as switches enabling the selection of the cell, the storage of a data in the capacitor and the displaying by the cell of a video information corresponding to the stored data;        a row or gate driver that selects line by line the cells of the matrix in order to refresh their content;        a column or source driver that delivers the data to be stored in each cell of the current selected line; this component receives the video information for each cell; and        a digital processing unit that applies required video and signal processing steps and that delivers the required control signals to the row and column drivers.        
Actually, there are two ways for driving the OLED cells. In a first way, each piece of digital video information sent by the digital processing unit is converted by the column drivers into a current whose amplitude is proportional to the video information. This current is provided to the appropriate cell of the matrix. In a second way, the digital video information sent by the digital processing unit is converted by the column drivers into a voltage whose amplitude is proportional to the video information. This current or voltage is provided to the appropriate cell of the matrix.
From the above, it can be deduced that the row driver has a quite simple function since it only has to apply a selection line by line. It is more or less a shift register. The column driver represents the real active part and can be considered as a high level digital to analog converter. The displaying of video information with such a structure of AM-OLED is the following one. The input signal is forwarded to the digital processing unit that delivers, after internal processing, a timing signal for row selection to the row driver synchronized with the data sent to the column drivers. The data transmitted to the column driver are either parallel or serial. Additionally, the column driver disposes of a reference signalling delivered by a separate reference signalling device. This component delivers a set of reference voltages in case of voltage driven circuitry or a set of reference currents in case of current driven circuitry. The highest reference is used for the white and the lowest for the black level. Then, the column driver applies to the matrix cells the voltage or current amplitude corresponding to the data to be displayed by the cells.
In order to illustrate this concept, an example of a voltage driven circuitry is described below. Such a circuitry will also used in the rest of the present specification for illustrating the invention. The driver taken as example uses 8 reference voltages named V0 to V7 and the video levels are built as shown below:
Video levelGrayscale voltage levelOutput voltage0V7 0.00 V1V7 + (V6 − V7) × 9/11750.001 V2V7 + (V6 − V7) × 32/11750.005 V3V7 + (V6 − V7) × 76/11750.011 V4V7 + (V6 − V7) × 141/1175 0.02 V5V7 + (V6 − V7) × 224/11750.032 V6V7 + (V6 − V7) × 321/11750.045 V7V7 + (V6 − V7) × 425/1175 0.06 V8V7 + (V6 − V7) × 529/11750.074 V9V7 + (V6 − V7) × 630/11750.089 V10V7 + (V6 − V7) × 727/11750.102 V11V7 + (V6 − V7) × 820/11750.115 V12V7 + (V6 − V7) × 910/11750.128 V13V7 + (V6 − V7) × 998/1175 0.14 V14V7 + (V6 − V7) × 1086/11750.153 V15V60.165 V16V6 + (V5 − V6) × 89/10970.176 V17V6 + (V5 − V6) × 173/10970.187 V18V6 + (V5 − V6) × 250/10970.196 V19V6 + (V5 − V6) × 320/10970.205 V20V6 + (V5 − V6) × 386/10970.213 V21V6 + (V5 − V6) × 451/10970.221 V22V6 + (V5 − V6) × 517/10970.229 V. . .. . .. . .250V1 + (V0 − V1) × 2278/30292.901 V251V1 + (V0 − V1) × 2411/30292.919 V252V1 + (V0 − V1) × 2549/30292.937 V253V1 + (V0 − V1) × 2694/30292.956 V254V1 + (V0 − V1) × 2851/30292.977 V255V0 3.00 V
A more complete table is given in Annex 1. This table illustrates the output voltage for various input video levels. The reference voltages used are for example the following ones:
ReferenceVoltageVn(Volts)V03V12.6V22.2V31.4V40.6V50.3V60.16V70
Actually, there are three ways for making colour displays:                a first possibility illustrated by FIG. 1 is to use a white OLED emitter having on top photopatternable colour filters; this type of display is similar to the current LCD displays where the colour is also done by using colour filters; it has the advantage of using one single OLED material deposition and of having a good colour tuning possibility but the efficiency of the whole display is limited by the colour filters.        a second possibility illustrated by FIG. 2 is to use blue OLED emitters having on top photopatternable colour converters for red and green; such converters are mainly based on materials that absorb a certain spectrum of light and convert it to an other spectrum that is always lower; this type of display has the advantage of using one single OLED material deposition but the efficiency of the whole display is limited by the colour converters; furthermore, blue materials are needed since the spectrum of the light can only be reduced by the converters but the blue materials are always the less efficient both in terms of light emission and lifetime.        a third possibility illustrated by FIG. 3 is to use different OLED emitters for the 3 colours red, green and blue. This type of display requires at least 3 material deposition steps but the emitters are more efficient since not filtered.        
The invention is more particularly adapted to the displays of FIG. 3. It can be also used for the other types of display.
The use of three different OLED materials (one par colour) implies that they all have different behaviours. This means that they all have different threshold voltages and different efficiencies as illustrated by FIG. 4. In the example of FIG. 4, the threshold voltage VBth of the blue material is greater than the threshold voltage VGth of the green material that is itself greater than the threshold voltage VRth of the red material. Moreover, the efficiency of the green material is greater than the efficiencies of the red and blue materials. Consequently, in order to achieve a given colour temperature, the gain between these 3 colours must be further adjusted depending on the material colour coordinates in the space. For instance, the following materials are used:                Red (x=0.64; y=0.33) with 6 cd/A and VRth=3V        Green (x=0.3; y=0.6) with 20 cd/A and VGth=3.3V        Blue (x=0.15; y=0.11) with 4 cd/A and VRth=3.5V        
Thus a white colour temperature of 6400° K (x=0.313; y=0.328) is achieved by using 100% of the red, 84% of the green and 95% of the blue.
If one driver with only one set of reference signals (voltages or currents) for the 3 colours is used and if the maximum voltage to be applied to the cells is 7 Volts (=Vmax), the voltage range must be from 3V to 7V but only a part of this dynamic can be used and all corrections must be done digitally. Such a correction will reduce the video dynamic of the whole display. FIG. 5 illustrates the final used video dynamic for the 3 colours. More particularly, the FIG. 5 shows the range used for each diode (colour material) in order to have proper colour temperature and black level. Indeed, the minimum voltage Vmin (=V7 in the previous table) to be applied to the diodes must be chosen equal to 3V to enable switching OFF the red diode and the lowest lighting voltage (=V7+(V6−V7)×9/1175 in the previous table) must be chosen according the blue threshold level to adjust black level. The maximum voltage to be chosen for each diode is adapted to the white colour temperature that means 100% red, 84% green and 95% blue. Finally, it can be seen that only a very small part of the green video range is used.
Since the video levels between 3V and 7V are defined with 256 bits, it means that the green component is displayed with only a few digital levels. The red component uses a bit more gray level but this is still not enough to provide a satisfying picture quality.
A solution is disclosed in the European patent application 05292435.4 filed in the name of Deutsche Thomson-Brandt Gmbh. In this application, a different reference signalling is used to display each of the three colour components. In this solution, the luminous elements are addressed in a way different from the standard addressing.
FIG. 6 illustrates the standard addressing of video data in an AMOLED display. The matrix of luminous elements comprises for example 320×3=960 columns (320 columns per colour) C0 to C959 and 240 rows L0 to L239 like a QVGA display (320×240 pixels). For the sake of simplicity, only 5 rows L0 to L4 and 5 columns C0 to C3 and C959 are shown in this figure. C0 is a column of red luminous elements, C1 is a column of green luminous elements, C2 is a column of blue luminous elements, C3 is a column of red luminous elements and so on. Each output of the row driver is connected to a row of luminous elements of the matrix. The video data that must be addressed to the luminous element belonging to the column Ci and the row Lj is expressed by X(i,j) wherein X designates one of the colour components R, G, B. The video data of the picture to be displayed are processed by a signal processing unit that delivers the video data R(0,0), G(1,0), B(2,0), R(3,0), G(4,0), B(5,0), . . . R(957,0), G(958,0), B(959,0) for the row of luminous elements L0 and the reference voltages to be used for displaying said video data to a data driver (or column driver) having 960 outputs, each output being connected to a column of the matrix. The same set of reference voltages is used for all the video data. Consequently, to display colours, this standard addressing requires an adjustment of the reference voltages combined with a video adjustment of the three colours but these adjustments does not prevent from having a large loss of the video dynamic as shown in FIG. 5.
The solution presented in the above-mentioned European patent application 05292435.4 is a specific addressing that can be used in a standard active matrix OLED. The idea is to have a set of reference voltages (or currents) for each colour and to address three times per frame the luminous elements of the display such that the video frame is divided into three sub-frames, each sub-frame being adapted to display mainly a dedicated colour by using the corresponding set of reference voltages. The main colour to be displayed as and the set of reference voltages change at each sub-frame.
For example, the red colour is displayed during the first sub-frame with the set of reference voltages dedicated to the red colour, the green colour is displayed during the second sub-frame with the set of reference voltages dedicated to the green colour and the blue colour is displayed during the third sub-frame with the set of reference voltages dedicated to the blue colour.
A little bit different solution is explained in more detail in reference to FIG. 7 that illustrates a possible embodiment. During the first sub-frame, the three components are displayed using the reference voltages adapted to the green component to dispose of a full grayscale dynamic for this component. {V0(G), V1(G), V2(G), V3(G), V4(G), V5(G), V6(G), V7(G)} designates the set of reference voltages dedicated to the green component. The two other components are only partially displayed. So the sub-picture displayed during this sub-frame is greenish/yellowish. During the second sub-frame, the green component is deactivated (set to zero) and the voltages are adapted to dispose of a full dynamic for the red component by using the set of reference voltages dedicated to the red component {V0(R), V1(R), V2(R), V3(R), V4(R), V5(R), V6(R), V7(R)}. The sub-picture displayed during this sub-frame is purplish. Finally during the third sub-frame, the green and red components are deactivated (set to zero) and the voltages are adapted to dispose of a full dynamic for the blue component by using the set of reference voltages dedicated to the blue component {V0(B), V1(B), V2(B), V3(B), V4(B), V5(B), V6(B), V7(B)}.
It is thus possible to adjust the 8 reference voltages (or currents) at each sub-frame. The only particularity is that the lowest reference voltages must be kept equal to the lowest threshold voltage of the three colours. Indeed, displaying a blue component means having red and green components equal to zero, which means equal to V7 that is the lowest reference voltage. So, this voltage must be low enough to have them really black. In the example of FIG. 5, we must haveV7(R)=V7(B)=V7(G)=VRth.
The only additional requirement is the necessity of addressing the matrix three times faster.
FIGS. 8 to 10 illustrates the functioning of the display device during the three sub-frames. In reference to FIG. 8, during the first sub-frame, the video data of the picture to be displayed are converted into voltages to be applied to the luminous elements of the matrix by the data driver that uses the set of reference voltages dedicated to the green component. The set of reference voltages are distributed between 3 volts (=V7(G)=VRth) and about 4 volts=V0(G) that is the maximum voltage that can be used for displaying the green component.
An example of reference voltages for the green component is given below:
ReferenceVoltageVn(Volts)V04V13.85V23.75V33.45V43.2V53.1V63.05V73
In reference to FIG. 9, during the second sub-frame, the video data of the picture to be displayed are converted into voltages to be applied to the luminous elements of the matrix by the data driver that uses the set of reference voltages dedicated to the red component. The video data corresponding to the green and red components are set to zero. The set of reference voltages are distributed between 3 volts (=V7(R)=VRth) and about 5.4 volts=V0(R) that is the maximum voltage that can be used for displaying the red component.
An example of reference voltages for the red component is given below:
ReferenceVoltageVn(Volts)V05.4V15.08V24.76V34.12V43.48V53.24V63.13V73
In reference to FIG. 10, during the third sub-frame, the video data of the picture to be displayed are converted into voltages to be applied to the luminous elements of the matrix by the data driver that uses the set of reference voltages dedicated to the blue component. The video data corresponding to the green component are set to zero. The set of reference voltages are distributed between 3 volts (=V7(G)=VRth) and about 7 volts=V0(B) that is the maximum voltage that can be used for displaying the blue component.
An example of reference voltages for the blue component is given below:
ReferenceVoltageVn(Volts)V07V16.46V25.93V34.86V43.8V53.4V63.21V73
In a more general manner, the colour component having the highest luminosity capabilities (in the present case, the green component) is displayed only in the first sub-frame. The colour component having the lowest luminosity capabilities (in the present case, the blue component) is displayed in the three sub-frames and the colour component having in-between luminosity capabilities (in the present case, the red component) is displayed during two sub-frames.
A drawback of this solution is that it requires addressing the matrix three times faster than a standard addressing. Another drawback is that there is some colour lag on moving edges since different colours are displayed at different time periods (for example Red+Green+Blue during the first sub-frame, Red+Blue during the second sub-frame and only blue during the third sub-frame)